1. Field of the Invention
The present invention relates to a fixed bed reactor for gas-phase catalytic oxidation and processes for producing acrolein or acrylic acid using the fixed bed reactor.
2. Description of the Prior Art
Fixed bed reactors in which reaction is carried out by allowing a gas containing a starting material compound to flow in a reaction tube(s) filled with a catalyst are heretofore widely used for gas-phase reaction. Examples of the fixed bed reactors used for gas-phase reaction may include multi-tubular reactors in which a catalyst is filled into many reaction tubes having a small diameter and insulated reactors in which a catalyst is filled into a single reaction tube having a great diameter. Even if any of these reactors is used, when reaction is continuously carried out, solid organic substances and carbides (these substances being hereinafter referred to collectively as the “catalyst inhibitor”), which are generated due to impurities contained in a feed gas, by-products produced by the reaction, and the like, are deposited on a catalyst to deteriorate catalyst performance and increase a pressure loss, thereby lowering the yield of a final product. Thus, it is necessary to restore the catalyst by periodically removing such a catalyst inhibitor through burning or the like.
As a method of restoring a catalyst, there is known a method in which the catalyst is taken out of the reaction tube(s) and then restored outside the reaction tube(s), but it is preferred to restore the catalyst inside the reaction tube(s) in view of working time for taking out the catalyst, re-filling the catalyst, and the like.
As a method of restoring a catalyst inside the reaction tube(s), for example, Japanese Patent Laid-open Publications No. 6-262081 and No. 6-263689 disclose methods of restoring a catalyst safely and efficiently, with the catalyst remaining filled in the reaction tube(s), by heat treatment at a prescribed temperature while allowing a mixed gas containing molecular oxygen and steam to flow in the reaction tube(s).
However, in these methods, the catalyst can certainly be restored without taking out of the reactor, but it is necessary to stop the reaction every time the catalyst is restored.
Therefore, there is required a method which makes it possible to carry out a stable continuous operation over a long period of time, not by periodically stopping the reaction and removing the catalyst inhibitor deposited on the catalyst, but by suppressing the deposition of the catalyst inhibitor itself.